Oil shale is a term used to refer to sedimentary rock compositions typically comprised of layers of clay and sand mixed with other inorganic compounds including, for example, calcium carbonate, calcium magnesium carbonate, and iron compounds. Also within this sedimentary rock are dispersed pockets of complex organic compounds known as “kerogen.” If the oil shale is heated, typically between 600 and 1000 degrees F., the kerogen is pyrolyzed to produce various carbonaceous petroleum products including, for example, oil, gas, and other residual carbon products. Similarly, oil or tar sands are types of naturally occurring bitumen deposits within sand or clay.
Typically, processing for the recovery of carbonaceous products from oil shale (or oil/tar sands) is divided into one of two general categories, above-ground processing or in ground (in situ) processing. Above ground processing involves the physical mining of the oil shale rock and its subsequent processing above ground to obtain the desired hydrocarbonaceous products. In contrast, in situ processing includes heating the oil shale rock underground in order to pyrolyze the kerogen and bitumen materials to produce hydrocarbonaceous products from the rock in situ. These hydrocarbonaceous products are then collected and further processed above ground. Historically above ground processing is typically more efficient because a high percentage of the kerogen contained in the mined rock is processed, it is also more expensive due to the process of physically mining the rock and bringing it to the surface or extensive strip mining for processing. Such above ground processing is also detrimental to the environment because of the displacement of significant amounts of rock, and environmental contamination due to the mining process whether in the form of dust, tailings, and/or groundwater contamination. Moreover, mining is notoriously dangerous. Conversely, in situ processing is less expensive because the rock is not mined, but rather processed in place. However, to date in situ processing has been less efficient at producing the hydrocarbonaceous products from the rock, which requires significant penetration through the rock by the processing heat, and the subsequent diffusion of the hydrocarbonaceous products back through the rock for collection.
Many prior art in situ processes also use “rubilization” or the breaking up of the oil shale formation to increase its permeability. Rubilization is typically conducted by generating underground explosions that are both expensive and potentially detrimental to the environment. For example, while rubilization can lead to increased permeability within the rock formation, which in turn permits improved flow of gases and liquids within the rock, rubilization can also complicate the extraction process by giving the carbonaceous gases and liquids alternate paths of escape, resulting in lower extraction yield as well as potential environmental contamination. As such it is desirable to avoid rubilization.
U.S. Pat. No. 4,928,765 to Neilson discloses in situ recovery of carbonaceous products from oil shale without rubilization. Neilson discloses placing a gas-fired heater assembly into a borehole within the oil shale formation. Once the gas-fired heater is lowered into the borehole, fuel gas and combustion air are introduced from above ground into the heater assembly, which is heated to between 1000 and 1500 degrees F. When the heater is maintained at those temperatures, heat radiates outward from the heater to create a cylindrical reaction zone within the oil shale formation. As the reaction zone reaches the desired temperature, the kerogen within the rock is pyrolyzed resulting in formation of natural gas, which is then extracted, brought to the surface, and further processed. As Neilson is a “closed system,” the combustion gases and exhaust gases are contained within the heater assembly, and are never mixed with the hydrocarbonaceous products, which are extracted from oil shale rock through a separate pipe from the borehole. However, the Neilson process has several drawbacks. First, the borehole Neilson used was large, typically on the order of 20 plus inches, which was necessary to allow the burner/heater to fit down the well, but which led to poor structural integrity of the borehole. Further, while an increase in oil shale heat transfer efficiency is produced above 1000° F., a significant increase in the loss of vertical structural integrity is also observed, especially in formations where large amounts of carbonate minerals are present. Also, control and management of the Nielson heater system was difficult and dangerous, particularly the feeding of the engine fuel and oxygen from the surface. Because of these drawbacks, Neilson was unable to utilize his system in wells below depths of about 100 feet. Further, while Neilson's process created a cylindrical reaction zone at the bottom of the borehole, the heat in the well dissipated quickly, thereby limiting the effective reaction zone to the area near the heater.
U.S. Pat. No. 7,048,051 to McQueen et al. and its progeny disclose a different “open system” approach where, instead of using a heater assembly within a borehole, processing gases are introduced directly into a borehole and used to create a conductive and radiant non-burning thermal energy front sufficient to convert the kerogen in oil shale or bitumen in oil sand into hydrocarbonaceous products. In this open system, the liberated hydrocarbonaceous gases diffuse back through the rock formation to the borehole where they mix with the processing gases, which are then extracted together from the borehole. Once outside the borehole, a variety of processes are used to recover the hydrocarbonaceous products from the processing gases. However, McQueen's method also has various drawbacks. For example, the McQueen process requires the capping and pressurization of the entire borehole and the maintenance of a sub-atmospheric pressure relative to the well inlet pressure to insure a positive flow of the combustion and product gases. Such a pressurized system requires precise control of the system pressure to avoid undesired backflow and possibly explosions. In addition, because the McQueen system is open, it is imperative to keep the inlet clear to permit the processing gases to continue to enter the borehole. However, during processing, rock and sediment from the sides of the borehole can fall into the bottom of the well (sluff) and block the processing gas inlet. In addition, the McQueen process requires an elaborate support structure above ground to support the weight of the system components within the borehole, yet permits for the substantial expansion of the system components within the well as the system is heated. Further, unlike the closed system of Neilson, the McQueen process mixes numerous undesired products from combustion gases and/or makeup gases with the product gases, which requires additional steps to manage.
Many prior art processes are directed to recovery of carbonaceous products from what has been termed the “mahogany layer” or “mahogany zone” of the oil shale, which can be found anywhere from near or on the surface to 2000 feet deep. This mahogany zone is a very rich deposit, typically having a Fischer assay of approximately forty-five gallons per ton or more. Both the Neilson and McQueen patents describe a system targeting all of the potential oil shale layers in a cylindrical payzone, not just the rich mahogany zone.
As such, an in situ process is desired that can target the mahogany zone, does not require sub-surface rubilization, that can be used in deeper wells without the structural, process control, and safety issues associated with open systems such as McQueen, and which does not require the separation of processing gases from extracted hydrocarbonaceous product gases.
In addition AMSO targeted the Illitic oil shale (a clay based oil shale) layer found below the nachrolite layers around 2000 ft. deep. This process, which went through several iterations, used two wells including a heater well and an adjoining recovery well. Along with most other in situ efforts, this process heated the kerogen and recovered the products through a recovery well once a reservoir developed. All efforst to create reservoirs have seen little success to date.